1. Field of the Invention
The invention relates to devices for cutting thin boards from squared timber and more particularly to a cutting device having a feed table and at least one cutting table with a cutting blade mounted thereon.
2. Description of Related Technology
A known device for cutting thin boards from a cant (i.e. squared timber) includes a feed table and at least one cutting table and is described, for example, in EP-A-497,233 (Aug. 5, 1992) and counterpart Canadian patent application 2,060,942 published Jul. 31, 1992. The cant which is to be cut into thin boards is disposed substantially lengthwise on the feed table and is fed toward the cutting table upon which a blade is mounted at a front edge thereof which faces the feed table. The cutting table supports a cant after a thin board is cut from the cant by the blade. The cutting edge of the blade is primarily disposed in substantially the same plane as the cutting table and is separated from and substantially parallel to the feed table. However, the cutting edge may project somewhat above the plane of the cutting table. A gap formed by the separation of the blade and the feed table is utilized for conveying a cut board between the cutting edge and a back edge of the feed table opposite the cutting edge. The distance between the cutting edge of the blade and the feed table largely determines the thickness of the board to be cut. The feed table and the cutting table are adjust:able relative to each other with respect to the gap therebetween so as to adjust for a desired thickness of a board to be cut.
The cutting blade includes two contact surfaces which form an acute angle at the cutting edge of the blade. One of the contact surfaces is disposed substantially parallel to the surface of the feed table and forms part of a cutting table upon which the cant (less a board thickness) remains supported after a board is cut from the cant. The other contact surface, which is disposed at an acute angle from the surface of the cutting table, substantially serves as a receiving surface for conveying the thin board being cut.
Such a cutting device includes a feed mechanism for pushing a cant forward on the feed table in a lengthwise direction across the feed table and against the cutting edge of the blade and pressing mechanisms for pressing a cant against the feed and cutting tables during the cutting of a board.
The feed table and the cutting table, insofar as it is connected to the blade's first surface, need not be solid surfaces. The two tables can contain, for example, rollers for conveying the cant. Another possibility, however, is that the feed table, with the exception of an edge abutting the oppositely disposed cutting edge and parts of the cutting table, is completely constructed as a roller conveyor.
In the type of cutting device described above which is of interest herein, a preferred configuration of cutting device elements is one in which the feed and cutting tables have substantially horizontal surfaces and a cut board is conveyed downwardly through the gap between the feed table and the cutting table. However, other configurations and alignments of the feed and cutting tables may also be utilized. For example, the feed and cutting tables may be arranged perpendicularly, whereby the cant is then pressed sideways on the two tables by means of a press mechanism. In known devices of this kind, the press mechanism includes a rotating rubber belt, which, by means of press rollers arranged on an inner side of the belt, is pressed against the cant. Feed mechanisms for conveying a cant through the cutting device include separated, power-driven feed rollers, which grip the cant in pairs on either side of the cant and draw or push the cant through the device.
It is evident that the removal of boards from a cant by cutting (as compared to other methods, such as sawing) requires on the one hand, the use of very large feed forces to pull the blade through the cant. On the other hand, relatively high feed rates, up to 150 meters/minute, are appropriate in order to operate at a cutting rate higher than the natural splitting rate of the wood. These requirements demand a very robust machine design.
The blades utilized in such a cutting device are subject to high wear and tear, not only because of the harsh working conditions, but also because of foreign objects present in the wood, which may damage the blade cutting edge. The harsh working conditions further necessitate that the blade be hard and stable and, moreover, that the blade and the blade support be screwed or bolted at an exact position. The down-time required to exchange a blade which has become damaged or dull may therefore be considerable. Depending on working conditions, the service life of such cutting blades can be on the order of only two hours.
In installations for cutting thin boards from squared timber having a high cutting capacity, several cutting stations or units are typically arranged in sequence. A cant passes through one cutting unit after another in order to cut thin boards from the cant. Depending on the capacity of the installation, a sufficient number of cutting units are arranged in sequence so that a cant is completely cut up into boards of predetermined thickness in a single pass through the series of cutting units. For example, seven cutting units can be arranged in sequence, whereby each of these cutting units constitutes a separate cutting machine. Alternatively, the cutting units may be arranged in groups within a single machine.
In such a linear arrangement of cutting units, if a blade of a single cutting unit has to be exchanged, it is typically necessary to shut down the entire line, even if a reserve cutting unit is available. In such a situation, the cants may be conveyed through a cutting unit in an idling mode without a board being cut from the cants. However, because the cants still pass through the region of the blade in an idling cutting unit, it is generally not possible to convey the cant through the unit and carry out blade-exchange work simultaneously. If blade exchanges of the individual cutting units of a line are required at different times, undesirable, above-average service times result for the line.